The present invention is directed to a method for the nondestructive testing of voltage limiting blocks and more particularly to determining the energy absorption capability of the block without destroying it.
Voltage limiting blocks, which are for example disk shaped and for many commercial applications 5 centimeters or more in diameter, are used in the commerical power field as lightning or surge arresters, for example, to protect large and expensive power transformers. A number of blocks are stacked together either in series or parallel. Ten to 20 blocks may offer a voltage protection of, for example, 35 to 50 kilovolts. The voltage limiting blocks are generally composed of zinc oxide doped with other metal oxides and are formed by sintering.
The problem in the prior art has been that of hot spots; i.e., locations of higher current density and therefore higher local temperatures. The existence of these hot spots has been recognized and that they were due to non-uniformities in the microstructure of the block itself. However, the only solution was to provide an energy absorption rating for a particular production run of blocks with a very large safety factor. The overall absorption rating, for example, in terms of joules per cubic centimeter was obtained only after the destructive testing of what was believed to be a representative sampling of the production run.
However, since at the present time the cause of nonuniformities or hot spots in voltage limiting blocks is not known the assumption that a few samples taken from a production run are representative of the remaining production run is very weak. Thus a large safety factor is necessary which in its commercial format means the cross-sectional area must be increased or more parallel stacks must be provided.